Study on the γ irradiation effects of long-wavelength HgCdTe photovoltaic detectors passivated by different structures has been carried out, one structure was a single layer of ZnS and the other was double layers of CdTe/ZnS. A real-time measurement of current-voltage (I-V) characteristic was performed during the irradiation process, and it was found that the detectors exhibited different irradiation effects. Numerical fitting of the resistance-voltage (R-V) curves of detectors was made according to the current mechanism of photovoltaic detectors. It was found that the current at larger reverse bias voltages was mainly trap-assisted-tunneling current(TAT), and was generation-recombination current (G-R) at smaller and zero reverse bias voltages. After an analysis of the R-V curves of detectors before and after irradiation, it was thought the double layers of CdTe/ZnS helped to reduce the irradiation displacement effect, and brought no obvious increase of indirect trap-assisted-tunneling current with irradiation dosage. It was also found that irradiation ionization effects were closely correlated to the initial material parameters of the detectors. A more obvious irradiation ionization effect was found for detectors with a single layer of ZnS which had a higher generation-recombination lifetime of minority carriers through fitting method, and the generation-recombination current showed a persistent increase with irradiation dosage.